Pulverulent material has been subjected to reduction of particle size in fluid energy mills for many years, but the expense of such treatment has rendered it impractical for all except certain limited applications.
Fluid energy mills rely on the introduction of particulate material into a vessel having a high-velocity, normally sonic or supersonic velocity, fluid medium recirculating therein. The circulating flow of fluid medium is normally used to effect a centrifugal separation of the particulate material to permit a withdrawal of the finely-ground material while the coarse material continues its recirculation. The coarse material is reduced in size either by impingement against other particles in the recirculating flow or else by impingement against the vessel walls. In the former case, there is considerable loss of energy in the prior art ways of causing the inter-particle impingement, and in the latter case, there is substantial erosion of the vessel walls due to the high speed impact of the particles against the walls.
Prior to the present invention, the fluid energy mills incorporated one or more of three basic designs, namely the "pancake", the opposed nozzle, and the tubular.
The "pancake" design consists of a short flat cylindrical vessel having tangential inlet nozzles for the fluid carrier medium and a central exhaust outlet. The inlet nozzles are designed to introduce jets of fluid medium into the chamber with an overlap between adjacent nozzles to impart a turbulent condition to the flow which assists the inter-particle impact within the flow. Commercially available mills of this character are normally designed for laboratory use and the flow from the jets carries the particulate material into abrading impact with the walls of the vessel, not only causing rapid deterioration of the vessel walls, but also tending to cause the particles to rebound in towards the center of the vessel where the coarse particles may be entrained in the flow of finely ground particles being carried from the mill through the exhaust port.
In the opposed nozzle mills, the particulate material is introduced into the mill with a jet oriented in one direction and the jet is impacted with a jet from an opposite direction to obtain maximum particle-to-particle impact at the junction of the jets. Although this type of mill avoids a substantial degradation of the vessel wall by the impact of particulate material, there is substantial energy loss through the use of the opposed jets. To assure maximum comminution of the particulate material in such apparatus, it frequently is combined with a "pancake" or a tubular mill.
In the tubular mill, the vessel is in the form of an upright annulus of a particular configuration and the circulation through the annulus is effected by jets disposed tangentially in the bottom portion of the annulus. A substantial part of the grinding effect is obtained in the zone where there is injection of additional jets into the recirculating flow of material, but heavy reliance upon the confinement of the flow by the vessel walls subjects the annular walls of the vessel to a substantial abrading action by the particle-laden fluid medium. As with the pancake mills, the random impact of the heavier particles against the walls of the vessel permits rebounding of these particles into the central outlet of the vessel with the result that the fine particulate material being discharged with the carrier medium is contaminated by the coarser particles which rebound into the discharged flow.